While, in general, photoelectric conversion elements, such as CCD (Charge Coupled Devices) sensors and CMOS (Complementary Metal Oxide Semiconductor) sensors, have been known as image-pickup devices that pick up images, recently, it has been proposed that thin-film transistors (hereinafter, “Thin Film Transistor” is abbreviated as “TFT”) are caused to function as photoelectric conversion elements in combination with capacitors or the like. As one example of such image-pickup devices, for example, there are image-pickup devices that include liquid-crystal display elements, TFTs that function as photoelectric conversion elements, and so forth for respective pixels disposed in a matrix, and that also include a backlight or frontlight which serves as a light source. The image-pickup devices are configured so as to be capable of performing information input by utilizing light incident onto the TFTs while the image-pickup devices perform image display by utilizing transmission of light from the light source through the liquid-crystal display elements (for example, see Patent Document 1). Because the image display and the information input can be performed in the same display region in the above-mentioned configuration, it is expected that the image-pickup devices are used as information input/output devices as replacements for touch panels.
When the above-described image-pickup devices of a display-function-integrated type are realized using a commonly known low-temperature polysilicon technique, it is considered that it is difficult to obtain accurate output values because it is impossible to disregard attenuation of signals that is caused by parasitic capacitances in panels. The reason is that a photocurrent, which occurs due to light irradiation, in the case of a low-temperature polysilicon (hereinafter, abbreviated as “p-Si”) is smaller than that in the case of an amorphous silicon (hereinafter, abbreviated as “a-Si”). Thus, in order to realize the image-pickup devices of a display-function-integrated type using the p-Si, a certain amplification function is necessary. As a specific example of such an amplification function, there is an amplification function of storing electric charge in capacitances, such as capacitors, in accordance with electric signals that are generated in the photoelectric conversion elements, converting the electric charge into voltages, storing the converted voltages in SRAMs (Static Random Access Memory), and outputting the voltages as digital values “1” or “0” (for example, see Patent Documents 2 and 3). According to the amplification function, because the SRAMs have also the amplification function, no attenuation of signals that is caused by parasitic capacitances in panels occurs. Furthermore, because the voltages are output as the digital values, output results of the voltages have an excellent noise tolerance.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-268615    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-292276    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-339640